Porosity evolution and mineral paragenesis during low-grade metamorphism of basaltic lavas at Teigarhorn, eastern Iceland

Abstract

Low-grade alteration of basaltic lavas at Teigarhorn, eastern Iceland, resulted in three distinct stages of mineral paragenesis that correlate to events in the burial and intrusive history of Icelandic crust. Metasomatism and brittle deformation during the paragenetic stages dramatically affected the paleo- hydrology of the lavas and formed temporally distinct mineral assemblages. The lavas initially contained up to 22 percent total porosity concentrated near the tops and bottoms of individual lava flows. Celadonite and silica (Stage I) precipi- tated along the walls of primary pores prior to deep burial of the lavas and occluded 8 percent of the initial porosity. During burial (Stage II), hydrolysis of olivine and basaltic glass led to the formation of mixed-layer chlorite/smectite clays in the matrix of the lavas and as rims filling 40 percent of the volume of primary pores. Chlorite contents in Stage II mixed-layer clay rims increased from 20 to 80 percent during the infilling of individual vesicles, reflecting increas- ing temperatures with time as the lavas were buried. The end of Stage II occurred after burial and is represented by filling of 40 percent of total primary porosity by zeolites (scolecite or heulandite stilbite mordenite epistilbite) and re- placement of plagioclase by zeolites and albite. The Stage II zeolite assemblages are indicative of two regional metamorphic mineral zones in eastern Iceland, the mesolite scolecite and heulandite stilbite zones. The presence of the bound- ary between the mesolite scolecite and heulandite stilbite zones indicates that the maximum temperature during burial metamorphism was 90° 10°C. Localized areas of intense hydrothermal alteration associated with intrusion of basaltic dikes (Stage III) overprint Stages I and II. Extensive fracturing and hydrothermal brecciation during Stage III added 3 to 11 percent total porosity in which mm- to cm-scale museum-grade crystals of quartz, calcite (Iceland spar), stilbite, scolecite, heulandite, and laumontite precipitated. Estimates of the tem- perature during Stage III (based on fluid inclusion homogenization temperatures in calcite, chlorite geothermometry, and the zeolite assemblage) range from 120° to 180°C. Although the thermobarometric conditions during Stages II and III led to similar mineral assemblages, careful attention to textural and geologic rela- tions permits seemingly complex, multi-stage parageneses in metabasalts to be interpreted in a petrotectonic context.